This invention relates to liquid metal cooled nuclear reactors of the pool kind.
In a liquid metal cooled nuclear reactor of the pool kind the fuel assembly is submerged in a pool of primary liquid metal coolant contained in a vessel housed in a concrete vault. The coolant system comprises a plurality of intermediate heat exchangers whereby heat exchange is effected between the primary coolant flowing through the fuel assembly and a secondary coolant which flows in heat exchange with water to raise steam and a plurality of coolant pumps which are carried by the roof of the vault and immersed in the pool of coolant. The heat exchangers are within a core tank surrounding the fuel assembly and the pumps are disposed outside the core tank. In operation of the nuclear reactor, primary coolant is drawn from the pool by the pumps and passed upwardly through the fuel assembly to the intermediate heat exchangers, the coolant then flows downwardly back to the pool through the tubes of the heat exchangers. Primary coolant flow through the intermediate heat exchangers is effected by a pressure head of coolant contained within the core tank and generated by the coolant pumps. There are several problems with a coolant system of this kind, in particular, the problem of severe thermal shock occasioned to an intermediate heat exchanger when there is an interruption in the flow of coolant in the relevant secondary coolant circuit, for example, as caused by a failure of the secondary coolant pump. During the transient, the primary intermediate heat exchanger inlet temperature front travels down the tube bundles of the heat exchanger and most of the heat transfer occurs in the region of a secondary inlet window at the base of the intermediate heat exchanger. The lower tube plate is therefore subjected to a temperature differential of about 200.degree. C. which produces high stress concentrations in the region of the secondary inlet window.
An object of the invention is to reduce the thermal shock occasioned to the intermediate heat exchanger of a liquid metal cooled nuclear reactor of the pool kind in such an emergency wherein there is an interruption in flow in the secondary coolant circuit.